using modern ashless or synthetic 2 stroke oils it will make no difference.
Running 40:1 in a carb designed for 50:1 will run a little rich so tend to build up carbon deposits a little earlier.
And 50:1 in a 40:1 carb will run a little lean, the power head will run a little hotter.
Unless you are using it for several hours at full speed it will not be a problem.
I run 25:1 mowers at 50:1 and have been doing so for over 30 years without any problems.
When 2 strokes became the devils engines destined to extinguish all life from the planet, companies like Stihl suddenly found their engines would run 100:1 quite happily.
Then the marketing department decided you could only do this if you used Stihl oil at $ 100 / gallon.
